1. Field of Invention
The invention relates to an organic electroluminescence device and, in particular, to an organic electroluminescence device with a low reflectivity.
2. Related Art
The organic electroluminescence device which is also named organic light-emitting diode (OLED) has the advantages of both LCD and inorganic LED, such as thin, light-weight, high-resolution, power-saving, self-emission and high-response, etc. However, the conventional OLED needs to be deposited a metallic electrode, the contrast of OLED panel will greatly reduce due to the reflection of the ambient light. The legibility for a conventional OLED panel is thus relatively poor.
A conventional method so far is to attach a layer of circular polarizer (CP) on an OLED by changing the phase of the incident ambient light to reduce the reflection. As described in the U.S. Pat. Nos. 5,596,246 and 6,211,613, the CP is attached on the display. This method has been adopted in mass production for OLED. However, the use of CP will increase the production cost. One may also use optical absorption and optical interference to reduce the reflectivity of the metallic electrode. The U.S. Pat. Nos. 6,185,032, 6,558,820, 6,545,409, and the early disclosure No. 2002/0043928 proposed another method, which coats a thin layer of dark-colored or black light-absorbing material before the reflective metallic electrode of the OLED. This method reduces the reflectivity of the metallic electrode and increases the panel contrast. Moreover, one can also use the method of using optical absorption along with the destructive interference to reduce the device reflectivity, as disclosed in the U.S. Pat. Nos. 6,411,019, 6,545,409, 6,429,451, and 6,608,333. The device structure disclosed in the U.S. Pat. No. 6,411,019 is to insert an interference layer in the OLED. The interference layer is between the electrode and organic electroluminescence (OEL) layer of the device to reduce the reflectivity due to optical absorption and destructive interference. To excite the OEL layer between the anode and the cathode, the inserted interference layer has to be made of a conductive material. The work function difference between the interference layer and the electrode has to be extremely small in order not to reduce the carrier injection and increase the device operation voltage. This reduces the selection of the interference layer materials. In order to satisfy both the conductivity and the work function requirements, one usually selects indium tin oxide (ITO), indium zinc oxide (IZO), or a mixture of aluminum and silicon oxide as the interference layer. The device structure disclosed in the U.S. Pat. No. 6,545,409 is to use the thin cathode/light-absorbing layer/dielectric layer/metal layer structure in the OLED and the cathode and the metal layer are electrically connected. By controlling the conditions of the light-absorbing layer, the dielectric layer and the thin cathode, the reflectivity is also reduced due to the optical absorption and destructive interference. However, it is necessary to use the light-absorbing layer with an absorption coefficient greater than 104 cm−1 and the dielectric layer. Thus, it is limited for choosing materials, and it is also a little complicated in manufacturing and condition controls.